This invention is directed generally to the field of determination of subsurface earth structure, and more particularly to determining the geometry and orientation of hydraulic fractures in the earth.
A significant portion of the world's oil and gas resource is contained in reservoir rocks that have high porosity but low permeability. These rocks are called "tight sands." An important technique for producing oil or gas from tight sands is hydraulic fracturing. Under high pressure, a fluid is pumped down the borehole, through the perforations at a selected depth, out into the reservoir rock. Generally, a vertical fracture is created in this rock on both sides of the borehole. The fracturing fluid would often be mixed with a proppant such as sand, to keep the fracture propped open after pumping is stopped. The large area of the fracture permits flow of oil or gas at commercial rates from the reservoir through the fracture and into the well.
In developing a tight-sand reservoir, it is usual that many holes are drilled. If each hole is drilled, completed and the fracture established, a great deal of cost is involved. Siting and spacing of these holes is critical to the economic recovery of oil and gas resources. In order to make optimum hole siting decisions, one must have knowledge of the azimuth of the fracture and of its geometry, such as length and height. Therefore, a need exists for an accurate method of estimating fracture length and fracture geometry from field measurements.